1. Field of the Invention
The present invention relates to fiber reinforced ceramic matrix composites and also to a method of manufacturing these composites. More particularly, the invention is directed to a fiber reinforced ceramic matrix composite having great reliability and enhanced initial cracking strength characteristics by bringing the constituent ceramic fibers into a braided structure and to a method of the manufacture of such ceramic matrix composite.
2. Description of the Related Art
Ceramic sintered bodies have found wide applications to electronic and structural parts for use in heavy electric components, aircraft components, automobile components, electronic components, precision machinery components, semiconductor device materials and the like. This is because such sintered bodies are generally less deteriorative in strength in the course of molding up to elevated temperatures and are excellent in hardness, electrical resistance, abrasion resistance, heat resistance, corrosion resistance, lightweight and the like as compared to conventional metallic materials.
Those ceramic sintered bodies, however, are smaller in tensile stress than in compression stress, and they are prone to cause undesirable brittleness, i.e. a phenomenon in which cracking readily proceeds under tensile stress. To render ceramics applicable to those parts for which high reliability is required, a keen demand has been voiced for the development of a ceramic sintered body having improved toughness and increased fracture energy.
More specifically, great reliability coupled with sufficient heat stability and high-temperature strength properties are required for ceramic structural parts for use as components of gas turbines, aircraft and automobiles. In order to meet these requirements, intensive researches have been made in an effort to commercialize a ceramic matrix composite (CMC) that could afford increased values of fracture toughness and fracture energy and sufficient thermal shock resistance and that could result from bringing composite materials such as fibers made of inorganic material or metal, whiskers, plates, particles and the like into a matrix sintered body in which the composite materials are held in a compositely dispersed manner.
In a fiber reinforced ceramic matrix composite formed by compositely dispersing the above ceramic fibers in a matrix sintered body, greatly improved cracking resistance is attainable. To increase this property, however, the fibers need to be incorporated in a relatively large amount into the matrix. Further, a preform derived or formed by weaving the ceramic fibers with each other in a plane or three-dimensional direction is employed as a fibrous structure so as to reduce anisotropy inherent to ceramic matrix composites.
In fabricating the aforementioned fiber reinforced ceramic matrix composite into various structural parts, it is desired that such composite be sufficiently great in initial matrix cracking strength, highly resistant to propagation of any crack arisen and moreover large in fracture energy.
In the case where stress applied or exerted to the ceramic matrix composite is lower than the initial matrix cracking strength, the composite so stressed is elastically deformable and substantially free from damage such as cracking and the like. Under these stress conditions, such composite offers to a full extent good environmental resistance peculiar to ceramics and involves no physical deterioration with time which would stem from propagation of damage taking place at an initial stage. Consequently, gaining the initial matrix cracking strength of a ceramic composite at a higher level is a significantly important requirement for designing parts. From standpoints of increased reliability and damage acceptability of the parts, the ceramic matrix composite should also importantly be great in its fracture energy.
However, high initial matrix cracking strength and large fracture energy are virtually difficult to simultaneously attain with conventional fiber reinforced ceramic matrix composites. That is, the resistance to crack propagation is smaller as the initial matrix cracking strength is higher with the result that the fracture energy is necessarily limited to a certain small value. Conversely, the initial matrix cracking strength tends to drop in the case of a conventional ceramic matrix composite having large fracture energy. The conventional ceramic matrix composites in any event fail to satisfy both initial matrix cracking strength and fracture energy in a well-balanced manner. This leaves the problem that they are applicable only for a largely limited range of fields.
As another drawback of the conventional ceramic composites, the strength characteristics and the like become objectionably anisotropic, depending upon the direction of orientation of fibrous structure-constituting fibers, thus leading to a part of physical instability as a whole. This is particularly true of a ceramic matrix composite having disposed in its ceramic matrix a fibrous structure resulting from laminating a plurality of fabrics one on the other. Such known ceramic matrix composite shows a sharp decline in interlaminar strength.
On the other hand, the ceramic fibers constituting the fibrous structure generally get easily impaired upon flexing or rubbing, resulting in breakage or fluffing of the fiber-constituting monofilaments. Hence, it is made difficult to provide a preform (fibrous structure) of a complicated shape such as a plane or three-dimensional fabric.